Water sprays have traditionally been used on continuous mining machines for several reasons. These include dust suppression, bit lubrication, and reduction of frictional ignition. These water sprays have always been located along the top edge of the miner boom, and sometimes underneath and on the sides of the boom. The top-mounted sprays were located in such a position solely for convenience of accessibility and maintenance. Underboom nozzles have not been practical because of either or both of the following: 1) underboom nozzles required significantly more maintenance due to the harsh conditions under the boom and are readily damaged; 2) by law, the machine boom must be blocked up physically and independently of the machine hydraulics in order to access the nozzles. This is a more labor-intensive procedure and, in actual mining, is unacceptable.
As has been proven by previous research, these top-mounted water sprays can frequently create more dust problems than they solve by creating a phenomenon termed "rollback". This rollback is most severe at marginal airflows in the mine entry .and with conventional cone-type water sprays operating at pressures as low as 40 psi.
In longer mine entries, this rollback usually occurs on the off-curtain side (side opposite the exhaust ventilation curtain). This is due to the lower air velocity on the off-curtain side created by the ventilation air short-circuiting diagonally across the continuous mining machine to the curtain. In short mine entries (as in turning a crosscut), the opposite will usually occur. This is due to the ventilation air "hugging" the off-curtain rib as it tries to turn into the entry. Thus, rollback can also be substantial in the area between the continuous mining machine and the curtain.
FIG. 1 shows a typical conventional top mounted water spray manifold (10) positioned behind the drum (12) on a continuous mining machine and supplied from the factory mounted on the machine. Several disadvantages are apparent as follows: 1) There are nozzles evenly distributed across the boom. The drawback to this arrangement is that when the machine is up against either rib, the sprays on the ends are creating significant turbulence on either face-rib corner. 2) There is a significant amount of overlap between spray patterns of nozzles, in particular adjacent nozzles. While this may be ideal from a coal-wetting point of view, it is quite contrary to the notion of trying to streamline the airflow at the cutting face. 3) All of the nozzles are identical and oriented perpendicular to the face. This suffers from the fact that the airflow that a given nozzle "sees" is not the same as the airflow "seen" by the adjacent nozzles.
For more than two decades, researchers have labored to develop practical, functional water spray systems for continuous mining machines. However, the result is usually that a spray system which works well for dust control in one machine position, e.g., the box cut, does not perform well at other positions such as the slab cut. One of the more successful water spray systems was developed by the United States Bureau of Mines and is known as the anti-rollback system. This system also employs top-mounted water sprays to apply virtually all of the water to the cutter drum. The design of this anti-rollback system, however, uses flat-fan sprays and orients the nozzles in such a way as to minimize the distance travelled by the water spray before impingement. This technique serves to minimize interaction time between the water spray and the ventilation air. It is noted that this system was designed primarily to reduce rollback-created operator dust exposure, not to entirely eliminate it. As a result, this system has reduced operator exposure to miner-generated dust by only 40 percent.
Another deficiency in previous water spray systems is that a water spray system designed for improved dust control usually is not effective in removing hazardous methane from the face area. Conversely, a spray system designed for efficient methane removal, such as the U.S. Bureau of Mines-developed Sprayfan system, would create significant dust rollback unless the spray parameters are carefully balanced with the face ventilation conditions. In actual practice this criteria is usually not met.